In recent years, digital signal-processing techniques have been introduced to electronic measuring instruments or video apparatus in addition to conventional analog signal-processing techniques. Especially, waveform memories, transient recorders, LSI testers treating both analog and digital signals, and other apparatus which convert analog signals from plural objects to be observed into digital form and record the digital data on a semiconductor memory or the like use multiplexer circuits for selecting one out of the output signals from plural apparatus.
A multiplexer circuit used for this purpose is generally required to have higher performance than the accuracy of the A/D converter connected at the next stage. Of course, a demultiplexer circuit which is opposite in function to the multiplexer circuit described above and divides the output signal from a single signal source into two or more for a desired number of apparatus is of importance similarly.
A multiplexer circuit is required to exhibit the following characteristics: When it is conducting, the ON resistance between the input and the output is low; The parasitic reactance of the switching portion is low, and the RF characteristics are excellent; The residual offset voltage is low; The dynamic range of the treated signal is wide.
On the other hand, the multiplexer circuit is required to exhibit the following characteristics when it is cut off: The OFF resistance is high when a signal is applied or delivered; The leakage current flowing in or out is small; The isolation between the input and the output is high. Preferably, the switching speed when a switch is made is high. Also, the driving voltage needed for switching is low, and the electric power consumed is low.
Taking account of the characteristics described above, relays or reed relays have been heretofore chiefly used for switching of DC signals. For switching of RF signals, mechanical relays such as coaxial relays have been predominantly employed. However, these devices have suffered from unsatisfactory lifetimes and reliability-because of their mechanical structure.
Meanwhile, multiplexer circuits using semiconductor switches have been developed in recent years. Typical semiconductor devices of this kind are discussed in IEEE transactions Microwave Theory And Techniques, Vol. 38, No. 2, pp. 109-117, February 1990.
FIG. 8 is a schematic showing the fundamental structure of a 1:2 demultiplexer circuit using semiconductor switches described in the above-described paper. A 1:N demultiplexer can be built by making a multiple stage cascade connection as shown in FIG. 9. In particular, the first stage consists of a demultiplexer circuit 71. Similarly, the second stage consists of demultiplexer circuits 72 and 73. The third stage consists of demultiplexer circuits 74-77.
In this figure, each of Q.sub.1 Q.sub.4 consists of an N-channel MESFET made of gallium arsenide. Each has such a characteristic that the drain current I.sub.D decreases with reducing the bias voltage V.sub.gs applied between the gate and the source as shown in FIG. 10. This is a so-called depletion characteristic. In this figure, when V.sub.gs =0, I.sub.D =I.sub.DSS.
In this example, Q.sub.1 and Q.sub.2 are FETs performing main switching action. The source terminals of these switching FETs act as output terminals OUT.sub.1 and OUT.sub.2, respectively. Q.sub.3 and Q.sub.4 are shunting FETs serving to improve the isolation between the two output terminals by short-circuiting the deactivated output terminals to ground.
For example, in FIG. 8, in order to cause the switch on the side of Q.sub.1 to conduct, the voltage V.sub.gs between the gate and the source of each of Q.sub.1 and Q4 is set to 0 V, and V.sub.gs between the gate and the source of each of Q.sub.2 and Q.sub.3 is biased negative more than the pinchoff voltage V.sub.p to bring the FETs into cutoff condition.
In the circuit described above, MESFETs made of gallium arsenide are used and, therefore, the amount of leakage current is relatively small. Also, the parasitic capacitance between electrodes is small. Especially, a demultiplexer or multiplexer in which the isolation between the output terminals at high frequencies is comparatively good can be accomplished.
Although the DC characteristics of the multiplexer circuit shown in FIG. 8 is relatively good, the voltage between the gate and the source is subject to modulation by the RF current applied via the parasitic capacitance between the gate and the source, for example, since the gate voltage of conducting Q.sub.1 and Q.sub.2 is maintained at a constant voltage. Therefore, the linearity between the input and the output deteriorates at higher frequencies. Hence, the output waveform is distorted.